1. Field of the Invention
The present invention relates to a method and a device for detecting deterioration of an exhaust purification catalyst.
2. Description of the Related Art
As a method of detecting the deterioration of an exhaust purification catalyst such as a three-way catalyst or the like, a method of employing the O2 storage operation of the catalyst is per se known.
For example, if the air/fuel ratio of the exhaust which is flowing in a three-way catalysts is within a comparatively narrow range centered upon the stoichiometric air/fuel ratio, it is possible to purify all the three components NOx, HC, and CO within the exhaust. Due to this, with a three-way catalyst or the like, by carrying an oxygen storage component such as cerium (Ce) or the like upon the three-way catalyst, it is possible to enhance the exhaust purification efficiency by imparting an O2 storage function to the catalyst.
An O2 storage function is a function in which an oxygen storage component which is carried upon the catalyst occludes the surplus oxygen component in the exhaust gas when the air/fuel ratio of the exhaust gas which is flowing into the catalyst is leaner than the stoichiometric air/fuel ratio, while emitting the oxygen component which has thus been occluded into the exhaust gas when the exhaust air/fuel ratio is richer than the stoichiometric air/fuel ratio. Due to this O2 storage function it becomes possible, even if the air/fuel ratio of the exhaust which is flowing into the three-way catalyst fluctuates more or less from the stoichiometric air/fuel ratio, still to maintain the ambient atmosphere around the catalyst in the neighborhood of the stoichiometric air/fuel ratio, so that thereby it becomes possible to perform exhaust purification efficiently with the three-way catalyst.
Such an O2 storage function decreases steadily along with deterioration of the catalyst. Due to this, it is possible to utilize the O2 storage function as a parameter of deterioration of an exhaust purification catalyst. When the O2 storage function decreases, the amount of oxygen which can be stored in the oxygen storage component decreases. Due to this, if the O2 storage function is normal, even if for example a state, in which the air/fuel ratio of the exhaust gas flowing into the catalyst is deviated from the stoichiometric air/fuel ratio, is maintained for quite a long time period, nevertheless, since the catalyst is able to occlude a sufficient amount of the oxygen from within the exhaust gas, or is able to emit a sufficient amount of oxygen into the exhaust gas, accordingly it is possible for the ambient atmosphere around the catalyst to be maintained near the stoichiometric air/fuel ratio.
In other words, if the O2 storage function is normal, even if the air/fuel ratio of the exhaust gas which is flowing into the catalyst (i.e. the catalyst upstream side exhaust air/fuel ratio) fluctuates more or less from the stoichiometric air/fuel ratio, still the air/fuel ratio of the exhaust gas after it has passed through the catalyst (i.e. the catalyst downstream side exhaust air/fuel ratio) is maintained in the neighborhood of the stoichiometric air/fuel ratio, and does not fluctuate.
However since, when the O2 storage function decreases, the amount of oxygen which is occluded in the oxygen storage component of the catalyst, or the amount of oxygen which is emitted from the oxygen storage component, decreases even under the same conditions, accordingly, after a state in which the catalyst upstream side exhaust air/fuel ratio fluctuates from the stoichiometric air/fuel ratio has been maintained for a comparatively short time period, then the catalyst downstream side air/fuel ratio also comes to fluctuate, in the same manner as the catalyst upstream side air/fuel ratio.
In Japanese Patent Application Publication No. JP-A-10-212935, for example, there is disclosed a method of detecting decrease of the O2 storage function of a catalyst, in other words deterioration of the catalyst, by detecting fluctuations of the air/fuel ratio on the downstream side of that catalyst. With this method of detecting deterioration of Japanese Patent Application Publication No. JP-A-10-212935, deterioration of the catalyst is detected based upon the ratio of the number of times, over a predetermined time period, that the catalyst upstream side exhaust air/fuel ratio changes between rich and lean (i.e. inverts), with respect to the number of times that the catalyst downstream side air/fuel ratio changes between rich and lean (i.e. inverts).
In, for example, the case in which the catalyst is not deteriorated (i.e. its O2 storage function is not decreased), even if the number of times that the catalyst upstream side exhaust air/fuel ratio inverts between rich and lean is great, due to absorption and emission of oxygen into and from the catalyst, the catalyst downstream side exhaust air/fuel ratio does not change very much, so that the number of inversions between rich and lean of the exhaust air/fuel ratio on the downstream side of the catalyst is quite small. Due to this, the value of the ratio of the number of times that the catalyst upstream side exhaust air/fuel ratio inverts with respect to the number of times that the catalyst downstream side air/fuel ratio inverts is large.
On the other hand, if the catalyst has deteriorated (i.e. if its O2 storage function has decreased), since the number of times that the catalyst downstream side exhaust air/fuel ratio inverts comes to approach the number of times that the catalyst upstream side exhaust air/fuel ratio inverts, accordingly the value of the above described ratio becomes small. In the invention of the abovementioned Japanese Patent Application Publication No. JP-A-10-212935, it is arranged to decide that the catalyst has deteriorated if the value of the above described ratio falls below a value which is determined in advance.
However, the above described number of inversions of the downstream side exhaust air/fuel ratio experiences a great influence, not only from the O2 storage function of the catalyst, but also due to the amount of exhaust which is flowing through the catalyst. In other words, since the amount of oxygen absorbed or emitted per unit time for the exhaust air/fuel ratio to approach the stoichiometric air/fuel ratio must be greater when the exhaust flow amount is great as compared to when it is small, accordingly, as described hereinafter, the fluctuations of the catalyst downstream side exhaust air/fuel ratio can easily become great even if the O2 storage function of the catalyst has not decreased, so that, even if the catalyst is still in its normal state, it may happen that an erroneous decision that the catalyst has deteriorated may still be arrived at.
It should be understood that while, in the abovementioned Japanese Patent Application Publication No. JP-A-10-212935, deterioration of the catalyst was decided upon using the ratio of the number of inversions of the air/fuel ratio between the upstream side and the downstream side of the catalyst, in any case, even if some other parameter than the number of inversions of the air/fuel ratio is employed, if the O2 storage function is used as a parameter of deterioration of the catalyst, there is a problem that the decision as to deterioration of the catalyst may become inaccurate if the exhaust flow amount is large. In order to prevent this, in the abovementioned Japanese Patent Application Publication No. JP-A-10-212935, the occurrence of mistaken decisions is prevented by not making a decision that the catalyst has deteriorated, if the exhaust flow amount is greater than a predetermined flow amount.
By, as in Japanese Patent Application Publication No. JP-A-10-212935 as described above, not making a decision that the catalyst has deteriorated if the exhaust flow amount is large, it is possible to prevent the mistaken decision that the catalyst has deteriorated when it is, in fact, normal.
However, if it is arranged not to make any decision at all as to deterioration of the catalyst if the exhaust flow amount is large, then, depending upon the pattern of operation of the engine, the opportunity for making a decision as to deterioration of the catalyst may be greatly decreased, and sometimes it may happen that no deterioration decision is made over a long period of time, irrespective of whether or not the catalyst has actually deteriorated, so that the problem arises that a catalyst which has deteriorated may remain in use.